1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to a so called microprocessor operated closed-loop control system in which the air-fuel ratio is adjusted in relation to the concentration of oxygen measured by a sensor in an exhaust system of the engine.
2. Description of the Prior Art
In an internal combustion engine provided with a catalytic converter to improve the emission characteristics, a closed-loop air-fuel ratio control system is generally utilized to produce an air-fuel mixture of a theoretical value (14.7:1) which allows the catalytic converter to work most efficiently.
Such a closed-loop air-fuel ratio control system comprises an oxygen sensor which is provided, in an exhaust system of the engine, upstream of the catalytic converter to produce an electric signal indicative of the oxygen concentration. The output signal of the oxygen sensor is then applied to a control circuit which produces a fuel supply control signal in accordance with the oxygen sensor output signal and other parameters which are measured by various sensors such as a throttle position sensor. The fuel supply control signal is applied to a fuel supply means such as a carburetor with an air-fuel ratio adjusting device. The air-fuel ratio of the mixture supplied to the engine is thus maintained at the theoretical value by the closed-loop control system.
The closed-loop control system, however, is designed to be capable of rapidly switching from its closed-loop mode of operation in which the air-fuel ratio is controlled to predetermined values other than the theoretical value, that is, independently of the exhaust emissions for meeting with the demands of good drivability and stability of the engine. For example, the closed-loop control system operates in an open-loop mode during start-up and cold operation of the engine and momentarily during acceleration and deceleration.
However, during the open-loop operation of the system, which is effected by fixing a needle valve for controlling the aperture of an air bleed in the carburetor for example, the air-fuel ratio tends to vary with the changes in atmospheric pressure and intake air temperature. In order to prevent such an adverse effect, the control circuit is generally provided with signals from an atmospheric pressure sensor and an intake air temperature sensor for producing an fuel supply control signal which is adjusted by the output signals form these sensors to compensate for the variation of the air-fuel ratio.
In prior art air-fuel ratio control systems operated by a microprocessor, however, the provision of the correction function, in other words, the provision of atmospheric pressure signal and the intake air temperature signal has required seperate programs for respectively processing the atmospheric pressure signal and the intake air temperature signal. Furthermore, such a provision has resulted in a slow down of the speed of processing in the microproccessor unit which might serve as the other engine control means at the same time. This condition has resulted in a deterioration of the preciseness of the air-fuel ratio control.